1. Field of the Invention
The present invention generally relates to electron beam projection lithography exposure tools and, more particularly, methods for aligning elements forming the electron beam projection column of such tools.
2. Description of the Prior Art
Demands for improved performance and increased manufacturing economy of semiconductor integrated circuits have driven minimum feature size regimes of integrated circuit designs to a small fraction of a micron. Such minimum feature sizes now being developed are beyond the resolution capability of lithographic exposure tools using deep ultraviolet (UV) wavelengths. Therefore, lithographic exposure with electron beams is now being developed and so-called probe-forming electron beam exposure tools have been in use for several years. However, probe-forming tools are only capable of exposing a single spot of perhaps as many as a few dozen or, at most, a few hundred pixels at a time. Modern integrated circuit designs, however, may include billions of pixels and sufficient throughput is not available from probe-forming tools to meet mass-manufacturing requirements even when individual exposures can be made at extremely high repetition rates.
Accordingly, electron beam projection lithography tools which feature patterning an electron beam with a reticle containing the pattern of a sub-field of an integrated circuit design and projecting the pattern on a target, usually with a demagnification of about 4:1 are being developed. However, the sub-field patterns must be projected with minimal distortion or aberration and must be positioned with an accuracy of only a small fraction of the minimum feature size design rules of the pattern to prevent, for example, narrowing of conductors which cross sub-field boundaries due to skewed positioning of sub-fields.
Electron beam projection lithography tools have numerous electron-optical elements in order to maintain aberrations at a very low level and to minimize aberrations while deflecting the electron beam as needed. Since the like charges of electrons in the beam cause mutual repulsion Coulomb effects, the electron beam column must generally be kept as short as possible. Therefore, the various electron-optical elements are often in such proximity that interactions occur. Each element may also have several distinct effects on the electron beam. For example, a focussing coil may cause beam rotation.
Aberrations usually increase with deflection and, for that reason, a translation stage is generally provided to assist the deflection in addressing all target locations. Similarly, to position respective reticle sub-fields to shape the beam, deflection elements and a translation stage are generally provided to move the reticle.
Additionally, the reticle must be sufficiently robust mechanically to withstand both accelerations necessary for rapid positioning and to avoid distortion and damage from thermal cycling due to electron energy absorbed when a portion of the beam is intercepted by the reticle pattern. Therefore, reticles are generally fabricated with relatively thick ribs running between patterned sub-fields formed by a much thinner membrane. The image of the shaping aperture at the reticle is adjusted in size to be slightly larger than the reticle sub-field to avoid hitting the ribs which could cause reticle heating and sub-field distortion.
Clearly, to achieve positional accuracy of a small fraction of the minimum feature size, many elements of diverse types must be exactly aligned and function together correctly. However, the variety of effects each element may have, the number of degrees of freedom for each element (e.g. axial position, axial rotation, axial alignment and the like) and the number of elements provided in an electron beam projection lithography tool (e.g. apertures, reticle, translation stages, deflectors, stigmators, focussing and collimating lenses and the like) have not allowed the development of a systematic methodology of tool element alignment which allows correct alignment to be achieved without a substantial number of iterations.
Additionally, there is no standardized configuration or order of elements in the electron beam column and when iterative adjustment is necessary, the element to be adjusted or the relative amount of adjustment to be made on two or more elements is not evident. That is, an artifact indicating a misalignment of one or more elements may not clearly indicate the element(s) which are misaligned. It is not even evident, when a particular adjustment is made, that other complementary adjustments will converge to a correct overall alignment of the elements of the tool.
It is therefore an object of the present invention to provide a systematic methodology for real time alignment of the reticle to the wafer in an electron beam projection lithography tool that avoids a need for writing of wafers.
It is another object of the invention to provide correct alignment of elements of an electron beam projection lithography tool, including sub-field magnification in a single iteration or very limited number of iterations.
In order to accomplish these and other objects of the invention, a method for aligning elements of an electron beam projection lithography tool is provided including centering and rotationally aligning an image of a shaping aperture with an image of a reticle sub-field to form a compound image, aligning orientation of the compound image with movement of a wafer stage of said electron beam projection lithography tool by lens adjustment, and correcting orientation and motion of the compound image relative to the movement of said wafer stage by rotational adjustment of a deflector. The above adjustments and adjustment of size and rotation can be performed in real-time by detecting a projected image at the target plane, preferably by scanning the image across a pin-hole and scintillator sensor and displaying a corresponding image.